The present invention relates to a magnetic recording medium and, more particularly, to a magnetic recording medium having a magnetic recording layer composed of magnetic crystal grains each having a column structure and grain boundaries.
As a main-stream recording system used in a hard disk drive, a longitudinal magnetic recording system which performs recording by orienting magnetization in a longitudinal direction of a substrate is used at present. However, a perpendicular magnetic recording system which orients magnetization in a direction perpendicular to a substrate has been studied vigorously to achieve a higher recording density, i.e., implement a larger-capacity hard disk drive. A recording medium used for perpendicular magnetic recording has an easy axis of magnetization in a direction generally perpendicular to a substrate and is composed of a magnetic recording layer for holding a record, a soft magnetic underlayer for the efficient use of a magnetic field from a magnetic head, and the like. In the perpendicular recording system, magnetic stabilization is achieved since magnetization is in an anti-parallel direction in a boundary portion (magnetization transition domain) between recorded magnetization domains (recording bits) and the width of a so-called zigzag domain wall is reduced so that medium noise is reduced. The soft magnetic underlayer not only serves as a return path for the magnetic field from the magnetic head but also can generate an intense magnetic field in the magnetic recording layer portion due to a mirror image effect when used in combination with a single pole head. This allows the use of a recording magnetic film with a high coercivity, also achieves a reduction in the width of the zigzag domain wall, and contributes to a reduction in medium noise.
As a condition on an extremely fine structure of a medium for a reduction in medium noise, it is requested that the grain size of each of magnetic crystal grains is extremely small and an exchange interaction between the adjacent crystal grains is small. The reason is that, since a unit of magnetic transition is one crystal grain composing the magnetic recording layer or a plurality of the crystal grains coupled to each other, the width of the magnetization transition domain is greatly dependent on the magnitude of the unit of magnetic transition. To greatly reduce the crystal grain size of the recording layer used in a perpendicular magnetic recording medium and reduce medium noise, in particular, there has been proposed a technology as shown in Japanese Unexamined Patent Publication No. 2003-115106, which changes an underlayer immediately under the recording layer into an amorphous alloy of Ti or Zr. Japanese Unexamined Patent Publication No. 2001-283428 discloses a technology which causes an element having a body-centered cubic lattice structure, such as Cr or V, to be solid-solved in Ru of the underlayer immediately underlying the recording layer and thereby greatly reduces the grain size. On the other hand, a magnetic recording medium having a so-called granular structure has been proposed in which the peripheral portions (crystal grain boundaries) around the magnetic crystal grains are surrounded by a non-magnetic layer for a reduction in the interaction between the crystal grains. For example, Japanese Unexamined Patent Publication No. 2002-358615 discloses a magnetic recording medium having a granular structure in which an average spaced distance between grains is adjusted to 1.0 nm or more. As a grain boundary layer to be used, an oxide, a nitride, a fluoride, a carbide, or the like is listed as examples. In addition, Japanese Unexamined Patent Publication No. 2003-178413 discloses a magnetic recording medium having a granular structure which uses an oxide for a non-magnetic layer composing a grain boundary layer, defines the volume of an oxide contained in a magnetic recording layer, and thereby implements a high coercivity. In the case of using a CoCrPt-based alloy in a longitudinal recording system, a non-magnetic element such as Cr is locally precipitated at the grain boundary portions so that the inter-grain interaction is reduced relatively excellently. If a CoCrPt-based alloy is used in a perpendicular recording medium, however, local precipitation of Cr at the grain boundaries is less likely to occur so that it has been difficult to form a satisfactory grain boundary.